Fuel cells have been projected as promising power sources for portable electronic devices, electric vehicles, and other applications due mainly to their non-polluting nature. Of various fuel cell systems, the polymer electrolyte membrane based fuel cell technology such as polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) have attracted much interest thanks to their high power density and high energy conversion efficiency. The “heart” of a polymer electrolyte membrane based fuel cell is the so called “membrane-electrode assembly” (MEA), which comprises a thin, solid proton conducting polymer membrane having a pair of electrode layers (i.e., an anode and a cathode) with dispersed catalysts on the opposing surfaces of the membrane electrolyte.
Proton-conducting membranes for PEMFCs and DMFCs are known, such as Nafion® from the E. I. Dupont De Nemours and Company or analogous products from Dow Chemicals. These perfluorinated hydrocarbon sulfonate ionomer products, however, have serious limitations in the application of high temperature hydrogen/air polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). Nafion® loses conductivity when the operation temperature of the fuel cell is over 80° C. Moreover, Nafion® has a very high methanol crossover rate, which impedes its applications in DMFCs. U.S. Pat. No. 5,773,480 assigned to Ballard Power System describes a partially fluorinated proton conducting membrane from α, β, β-trifluorostyrene. One disadvantage of this membrane is its high cost of manufacturing due to the complex synthetic processes for monomer α, β, β-trifluorostyrene and the poor sulfonation ability of poly (α, β, β-trifluorostyrene). Another disadvantage of this membrane is that it is very brittle, thus has to be incorporated into a supporting matrix.
U.S. Pat. Nos. 6,300,381 and 6,194,474 to Kerres et. al. describe an acid-base binary polymer blend system for proton conducting membranes. While these membranes can reduce methanol crossover and have certain flexibility, they have difficulties in adjusting the membrane mechanical properties due to the limitation of the binary system.
WO 01/94450, which is incorporated by reference herein, describes a proton conducting membrane with a ternary system, in which elastomeric polymers are added to provide high proton conductivity and low methanol permeability.
However, the need for a good membrane in the fuel cell operation requires balancing of various properties of the membrane. For instance, among other properties such as proton conductivity, water retaining ability is very important for high temperature applications, fast start up of DMFCs, and maintaining cell performance. In addition, it is very important for the membrane to retain its dimension stability over temperature. In the case of a DMFC, the methanol crossover and chemical reaction generate enough heat to raise the cell temperature. If the membrane swells significantly, it will cause more methanol crossover, thus a higher cell temperature. The membrane thus gradually loses its ability to block methanol crossover, resulting in degradation of the cell performance. The dimension changes of the membrane also put a stress on the bonding of the membrane-electrode assembly (MEA). Often time this results in delimination of the membrane from the electrode after excessive swelling of the membrane. Therefore, maintaining the dimension stability over temperature and avoiding excessive membrane swelling are important for the DMFC application.
In the prior art, crosslinking agents are often used to covalently link molecular chains together, such as crosslinked sulfonated polystyrene used for water purification. Crosslinking agents, however, often reduce proton conductivity and cause membrane brittleness. Therefore, it is desirable to develop a dimensional stabilizer to provide membrane dimension stability without significant loss of membrane conductivity.
Furthermore, the morphology of most prior art membranes are not maximally optimized to create a membrane structure that can provide high conductivity, low methanol crossover, and good dimension stability and water retaining ability at same time. Block copolymers are more advantageous over random copolymers due to their distinctive domain size and ability to retain most of their own characteristics in a polymer blend system. Block copolymers are also more efficient in adjusting the desired properties of the membrane. Therefore, it is desirable to develop a polymer blend membrane that is optimized with high conductivity, low methanol crossover, good dimensional stability, and good water retaining ability by using multi-component functional groups including block copolymers.